Divergent evolution

Adaptive Divergence

Evolution has been described as the process of gradual modification in the plants or animals (living organisms) basically two patterns are distinguished in the process of evolution. The minor changes in the gene pool of a population from one generation to the next may not produce new populations. The newly formed population is not genetically identical with its predecessor. This is called ‘Sequential evolution’. The changes occur in the newly evolved populations, species, families and classes is known as divergent evolution.

 

The animals of the same group are closely related groups exhibit great divergence in their morphology when they are found in different habitats. Prof. Osborn states that “each isolated region, if large and sufficiently varied in its topography, soil, climate and vegetation will give rise to a diverse fauna. The larger the region and more diverse the conditions, the greater will be varieties of animals found.” Therefore, the divergent evolution in specialized directions, starting from a common and generalised type or the entry of organisms of the original stock to new adaptive zones.

Example -1 The limb structure of placental mammals provides a classical example of divergent evolution. The ancestors of all the present day types of mammals can be traced back to a primitive insect eating five toed, short -legged creature walked with the soles of their flat feet. The pent dactyl limbs were not modified for any particular type of locomotion. These lived on land and formed as ancestors to the modern mammals. Now the modern mammals have occupied five different habitats. Therefore, divergence occurred in five lines for five different habitats with modification In their limb structure.

I) The first line lead to “arboreal (climbing) modification “seen tree-dwelling forms like squirrels and primates.

 

ii) The second line acquired “aerial (flying) modification”, found in animals adapted for flight (Bat)

iii) The third line represents “cursorial (running) modification”. This type of mammals are adapted to fast running - Horse, Deer’s, Dogs etc.

iv) The fourth line acquired ‘fossorlal (burrowing) modification’, seen in moles.

v) The last line lead to “aquatic (swimming) modification “found in seals, whales etc.

In all these lines, mammals exhibit the modified limb structure for some particular mode of locomotion, So these limb types are derived from one common type represented by short pentadactyl limbs of terrestrial mammals. This shows a relatively generalized ancestral group gives rise to many relatively more specialized descendents.

Example (2) : Mammals possess’ heterodont dentition.

The incisors for biting, canines for tearing and grasping and the premolars and molars suited for grinding.

the premolars and molars exhibit greatest structural modifications for different types of food.

I) Insectivorous type Insect feeders - modified for crushing feeble prey.

II) Carnivorous types: Meat eaters - modified by having high crowned with complicated cusps-carnasial.

UI) Herbivorous type modified for succulent vegetation & harsh grasses. Incisors are suited for cuffing the vegetation.

The toothed whales have become secondarily homodont with grasping teeth. In sperm - whales, the teeth are absent. This type evolution of group Is known as macro evolution.

Speciation in Organic Evolution

Speciation- Types-factors

A species is an ecological unit. The species is a group of potentially  interbreeding natural populations which share in a common gene pool. But species are reproductively isolated from other such groups. A species comprises of several populations. There are two distinct ways in which new species arise from the pre-existing ones.

 

i) Splitting of the species into two or more species is called Speciation.

ii) Transformations of the old species into a new one in due course of time is known as transformation in time.

It is very interesting to note that a species population usually has Discontinuous distribution. The species populations may in due course of time become separated by distance. Populations ‘A’ and ‘Z’ of a given species grow in size generation after generation. As such, the organisms radiate into progressively larger territory. After few generations these populations A & Z at the opposite ends of the territory may be too off. Thereby the direct gene flow among their members is not possible and they do not have reproductive contact directly.

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The distance isolation is most common. These are other kinds of geographic isolations. The groups of related organisms become separated by some physical barriers like Sea1 mountain, desert, river. Moreover, homing instinct and territoriality oL animals also add to the isolation of individuals into groups populations.

Different types of speciation: There are three different types of speciation

i) Allopathic speciation or Geographic speciation:

The allopathic species have originated from the geographically isolated populations of a species.

a) Geographic isolation of demes of widely spread out population of a species over a period of a number of generations.

b) Genetic differences are accumulated independently in each deme.

c) Genetic divergence leading to the establishment of reproductive isolation.

 

So in alloptiric speciation, the population of a species divides and evolve into two or more new species. The basic factor in alloptric speciation is the geographic isolation. So it is also known as geographic speciation.

Example In the Australian continent, a bird “Acanthiza pusilla” is widely distributed. It slightly differ from the Tasmanian species - ‘A. Ewingi’. It is presumed that during pleistocene glaciation, when the sea level is lower, Acanthiza entered Tasmania. With the rise of sea level it became isolated and differentiated into A. ewingi.

ii) Parapatric speciation White (1968-78) studied this type of speciation. He proposed that chromosomal aberration leads to partial reproductive isolation in the individuals of a population. Any chromosomal mutation which lowers fertility, gradually confers reproductive isolation in a very small population by genetic drift.

iii) Sympatric speciation: This type of speciation arises due to some biological barrier for interbreeding. Polyploidy in plants leads to sympatric speciation.

If the hybrid between the two diploid species becomes tetraploid, it will be reproductively isolated from its diploid parents. Because the triploid offspring’s produced by back crossing with high ratio of aneuploid gametes which fail to survive.

As a general rule, a species is composed of a number of allopatric breeding populations, which are physically separated from one another to pursue independent evolutionary path. Each population is exposed to a different selection pressure. Therefore, random mutations along with genetic drifts and selection pressure establish genetic difference. The morphological and physiological variations are also present in the similar populations. These differences gradually accumulate and cause more and more divergence in the genetic constitution of populations.

Finally distinct sub-species is established. The geographical races or subspecies given an opportunity to interbreed or crossed artificially these produce fertile hybrids. The genetic modifications appeared so far have not reached to the e4ent of producing reproductive isolation. These may still exhibit preference of mating to the members of their own group.

Addition of certain more variations in their gene pool so as to affect their interbreeding, leads to reproductive isolation. The groups or subspecies or races become reproductively isolated and are ranked as species. The establishment of reproductive isolation is an event of biological significance.

The process of speciation occurs only in allopatric populations. It cannot act on sympatric populations inhabiting the same area. The sympatric species can arise either due to changes in the chromosome number or due to introgressive hybridization. The change in chromosome number may occur by polyploidy, aneuploidy, haploidy or translocation.

Ex. Different species of ‘Drosophila’ have different number and Appearance of chromosomes.

 

The chromosomal composition of P. virilis is regarded to be the ancestral type. The chromosomal complement of 13. pseudo obscura and D. persimilis could tiave derived from D. virilis by a translocation between X chromosome and one of the autosomes. The chromosome compliment of D. melenogaster could be derived by two translocations between the two pairs of autosomes.

Transformation of species in Time:

In the second method of evolution, in due course of time the transformation of a species into a new species takes place. in this type of evolution only one species exists at a time. For example, species ‘A’ evolves into ‘B’ and ‘B’ into ‘C’ and so on.

Sympson has recognized two types of Transformations

i) Phyletic evolution ii) Quantum evolution

I) Phyletic evolution This type of evolution involves the sustained directional changes in the average characters of a population. This may be due to adaptations to a shifting environment or due to increasing specialization for a particular environment or improved adaptations in a constant environment. Thus the evolving forms present in a line of succession one being replaced by the other.It is presumed to lead to the origin of new genera and families.

ii) Quantum evolution It is related to the higher taxonomic groups like orders and classes. It involves rapid shift or sudden changes in the organization of a population to a new equilibrium. It is distinctly different from the ancestral forms and adapted to occupy new conditions. The quantum evolution is macro and mega evolutions operating above the species level.

ISOLATION

ISOLATION-its Role In Evolution

 

The division of a single population into two or more groups because of some barrier for interbreeding is called Isolation. There are a number of processes by which two related populations living in the same area, can remain distinct These have been called Isolating mechanisms by Dobzhansky. According to him, the isolating mechanisms are classified into the following types.

I) Geographical Isolation Two parts of one population are separated by some geographical barrier and arc prevented from interbreeding. Large bodies of water are barriers for land-dwelling animals. High Mountain ranges, deserts, dense forests and extremes of temperature serve as affective barriers. Such populations are completely ‘out of touch with each other genetically’ so that new mutations, genetic drift and the action of natural selection, in one population have rio effect on the other population. Thus, a new population may be developed.

ii) Environmental isolation: Population living under different environmental conditions remains isolated from one another and are prevented from interbreeding. Environmental isolation depends upon differences in food habits and other physiological requirements of the animals.

For example, an insect which inhabits only coniferous frees, is environmentally isolated from an Insect which inhibits only the deciduous trees.

iii) Seasonal isolation: The breeding season of two groups of animals or plants do not coincide..Th American toad. Bufo Americans, and the Fowler’s toad, B. fowleri have similar distribution and form fully fertile hybrids in the laboratory crosser. But in nature, they remain distinct because B. americanus breeds early in the season and B fowleri breeds late.

Iv) Mechanical isolation: - The anatomy of the reproductive organs different from each other that copulation between males of one population and females of the other, is impossible.

The genitalia of a male will fit into those of a female of the same species as a key fit into a lock, but will not fit the genitalia of females of other species.

Observations have not confirmed this theory.

v) Physiological Isolation: Reproductive isolation may exist in those cases in which matings - between the males and females different populations take place. Patterson has shown that in some interspecific matings in Drosophila, the sperm fails to survive in the receptacles of the female of other species.

vi) Hybrid Sterility: - Normal vigorous hybrids are formed but they are sterile and further exchange of genes is completely blocked. The Mule is a classical example of hybrid sterility.

Origin of isolating Mechanisms: According to Muller reproductive isolation is due to differences in genes that arise during the origin of sub-species and species in population.

According to Dobzhansky, reproductive isolation is the result of natural selection. Hybrids are either sterile or poorly adapted and are, therefore, eliminated by natural selection.

Isolation and Species formation: Two populations become seperated from each other by means of geographic environmental barriers; each acquires new mutations and is acted upon by forces like genetic drift, natural selection, etc.

When a gene pool becomes divided by some geographic environmental factors, the allotrophic populations become differentiated so as to give rise to reproductive isolation. After the development of reproductive - isolation, the populations may again come! into contact, still they remain distinct and &e said to be sympatric. Thus, geographic environmental isolation is this species formation. The development of the reproductive isolation brings new species formation.

Hardy-Weinberg Law

Hardy Weinberg Law of Equilibrium:

The most fundamental idea in a population genetics was proposed by English-man G.H. Hardy and German W. Weinberg simultaneously in the year 1908. Later in 1929-30, the mathematical treatment of the distribution of gene and genotype frequencies in a population was developed principally by R.A. Fischer, JR. Haldane and Sewall Wright. The Hardy-Weinberg Law is the foundation of population genetics and of modem evolutionary theory.

This law can be defined as ‘The relative frequencies oi various kinds of genes in a large and randomly mating sexual panmictic population tend to remain constant from generation to generation in the absence of mutation, selection and gene flow”.

 

Hardy-weinberg’s law describes a theoretical situation in which a population Is undergoing no evolutionary change. It explains that if the evolutionary forces are absent, the population is large and its individuals have random mating. Thus each parent produces equal number of gametes. Such gametes combine at random and the gene frequency remains constant. Finally the genetic equilibrium of the genes is maintained and the variability present in the population is preserved.

For example, suppose there is a panmictic population with gene ‘A’ will be the same is the frequency of gene ‘A’: Similarly the frequency of gametes with ‘a’ will be equal to the frequency of the gene ‘a’.

 

If the gametes unite at random, the total number of different genotypes will be

 

There is a random union of the gametes with gene ‘A’ and ‘a’ at the

Equilibrium state, the population will contain the following frequencies of the genotypes and genes ‘A’ and ‘a’ generation after generation.

AA + 2 Aa + aa (or) p2 + pq + 42 genotype frequency. in population of large size, the probability of receiving.

i) The gent ‘A’ from both this parents will be p c p = p2, ii) for gene ‘a’ will be q x q = q2

iii) the probability of being heterozygote will be pq + pq = 2 pq.

As such the relationship between gene frequency and genotype frequency can be expressed as

2

p +2pq+q =1

2

(p+q)

- It is known as Hardy-Weinberg formula or binomial expression. it is clear that in a large and randomly mating population not only gene frequency but also the genotype frequencies will remain constant.

According to Hardy-Weinberg’ law:

i) The gene and genotype frequencies of each allele in a population remain at an equilibrium generation after generation. -

ii) in a- population, the mating is a completely random- manner.

iii) The equilibrium in the genotype and gene frequencies occurs on in large sized populations. But in small sized population gene frequencies may be un predict table.

iv) MI the genotypes in a population reproduce equally and successfully,

Example In human populations, persons with gene T find weak solution of PTC (Phenyl -thio-carbamide) to be bitter in taste. But the homozygous ‘ft persons, the PTC is tasteless, Moreover, persons are unaware of their reaction to PTC and nobody selects his mate according to whether he or she can or cannot taste this substance. As such the marriages take place at random. Suppose, in a particular island or in a town the number of homozygous tasters (17) and of homozygous non tasters is equal, the probable marriages could occur as follows.

 

Therefore, the geno type frequency in the first generation will be TT-25% Tt = 50% and It = 25%. The homgous tasters (Ti’) and heterozygous tasters (Tt) are phenotypic ally alike. So the populations possess 75% tasters and 25% are non-tasters. The same results can be obtained if we consider the union of gametes at the time of fertilization.

 

There the genotype frequencies according to Hardy-Weinberg’s equation.

0.25 TT + 0.50 Tt.+ 0.25 ft (p - is frequency for gene 1)
p2 ÷2pq+q2 =l(q isfrequency for t )

 

This law provides a situation, where the genes in the population have reached the equilibrium and the gene pool is constant In such case, there will be no evolution. In nature, the mutations, natural selection, Non- random mating, genetic drifts and differential migration operate to change the genetic equilibrium actually can bring about organic evolution.

Genetic Drift

 

Genetic -drift as one of the evolutionary force

The genetic architecture of small population changes irrespective of elective advantage or disadvantage. Analogously genes attain Hardy- Weinberg’s equilibrium in large populations only. The random changes in gene frequencies occurring by chance and not under the control of natural selection are called genetic drift. A series of steps and at each step the movement made is random, in directed which Is known as stochastic process.

The theory of genetic drift was developed by a geneticist

SE WALL WRIGHT in 1930. It is also known as Sewall Wright effect or ‘scattering of variability’. It denotes that the random fluctuations in the gene frequencies In a small population from generation to generation.

 

In demes of limited sizes, random genetic drifts arise by chance. These cannot arise in large population. For example if we compare two populations of two extreme sizes - Population ‘A’ consisting of 5,000 breeding individuals and population ‘B’ of only 50, the gene pool of each contains equal number of Land I. If their gene frequencies are represented by p & q

pL= qI -0.5

In the next generation, the gene frequency is expected to deviate from the original 0. 5 by an amount of equal to the ‘standard error’

The standard error is determined as the sequence root of the product of original frequencies (p & q) divided by the number of genes available. This number of genes will be double the number of breeding organisms.

In small population B with organisms, the standard error would be J(o. 5 x 0.5 + 1oo) = 0.05. So in the next generation the gene frequencies will change to 0.45 and 0.55 either way. This amounts to 10% change in the gene frequency. Thus the standard error in a large population’s) + 10,000 = 0.0005) is negligible and it significantly high in a small population.

 

Effects of Genetic Drift on Gene frequency

In small populations or demes, the genetic drifts have the following, effects on the gene-frequency. -

I) Homozygocity In small populations, due to genetic drift gene frequencies continue to fluctuate until one of the allele lost and other fixed. This leads homozygosity in small populations. It means the genetic drift reduces genetic variability by eliminating one of the two alleles ‘either new or old one.

ii) Fixation of new mutations Since genetic drift tend to eliminate one allele and fix the other one, irrespective of its dominance or recessiveness or advantageous or non advantageous nature. So a new mutation has 50% chances of either being lost or be fixed in small population.

 

iii) Genetic divergence: The demes become progressively genetically different. In each sub population, the genes fixed and lost will be different. Thus, In due course of time, (each deme gradually diversifies from the other sister demes) lead to the establishment of new species.

Genetic Drift and Evolution:

The role played by the genetic drifts actually in the evolution of organisms in nature is doubtful. A widely ranging broad base population is isolated into small sub groups - ‘DEMES’. The causes for isolation may be either on account of ecological or geographical discontinuities, home instinct. The size of these small demes is such that they appear to be affected by chance of events underlying genetic drift. The limited size of small breeding populations, the gene pool of their new generations may not be the same of the parental gene pools due to the action of genetic drift. The changed gene pools gradually lead to the formation of new species.

Founder Effect: Whenever a few organisms from large population encroaches a new or isolated geographical region, these form the “founders or ‘founder members’. The founders carry only a limited portion of the parental gene pool. The descendants of the founder i.e. the founder population or marginal isolates in a new area will tend to have ratios similar to the founders. The resemblance of the descendants of the founders is called founders effect’ or ‘founder principle’ (Maw).

- The diffusion of genes into populations through migrations and interbreeding is known as Gene flow. The gene flow links all the demes of a population. It tends to counteract the loss of variability due to genetic drift in small population.

Rh gene {r) was introduced into the Chinese population by American immigrants. This Rh factor is associated with erythroblastosis fetalis or hemolytic disease in new born.

Gene Mutation

Gene Mutation -Point Mutation

Now a days the word mutations is related to the gene mutations, If a nucleotide sequence of DNA molecule undergoes a change, it is called mutation. That gene is called mutant gene. In 1909 Morgan observed white eyed Drosophila. This white eye color is a mutant character. The mutant gene and original gene will be present at the same locus of the chromosome. The mutant gene Is always recessive. Only in homozygous condition the mutant gene will be expressed. Gene mutations are of two types. In nature mutations arise by themselves. They are called spontaneous mutations.

 

Induced Mutations: By the applications of mutagenic agents mutations are brought forward. This l& called Induced mutation. In 1927 Muller brought’ induced mutations by X-rays in Drosophila. For this he awarded noble prize in 1928 Stadler induced mutations in Barley plant by X-rays. Auerbach used mustard gas to bring mutations. Many chemicals will cause mutations.

Reverse Mutation: A normal gene will undergo mutation and become a mutant gene. After some time this mutant gene will undergo mutation and becomes original gene. This is called reverse mutation.

Eg: in a a wild pea plant white flower is because of original gene. Because of mutations it becomes purple. Because of reverse mutation it becomes white

 

Characters of Gene Mutation:

• Mutation occurs at random. A direction cannot be predicted.
• It will not occur to full fill a specific requirement.
• The mutation is because of change.
• Usually the mutant genes are recessive.
• Usually the mutations are harmful.
• Mutations will form a raw material for evolution.
• DNA is the basic material of a gene. DNA is responsible for hereditary characters. In a DNA molecule a specific base sequence is present. It will not change through many generations.

A DNA molecule is made by two polynucleotide chains. In each strand four kinds of nucleotides are present. The genetic property of particular genetic molecules of DNA is determined by the specific sequence of its nucleotide bases. Changes In this sequence or on base pair is changed, mutation is caused. Watson and Crick predicted that base pair change at any point But it is very rare. This Is represented in the following way. AT, is changed into G.C. pair.

This type of change is called mutations. This is carried on to a point. Hence it’s called Point mutation. Because of this it is invisible, it is called Invisible mutation. This is also called Gene mutation. This is brought forward by following methods.

In DNA the base pairs are A-T and 0 - C in each of the four base molecules various atoms are arranged in a certain manner. A single shift in this arrangement as a result of hydrogen atom moving from one position to another position, within molecule will cause the change in the form of base. This process i called Tautomerism. The two forms of a base are called tautomeres common tautomeric forms of 0 and T are “keto” compounds The tautomeric forms of A and C are “amino” compounds. Now and then “ionyl” forms, “amino” forms may occur. These are called abnormal tautomers. Because of these abnormal tautomers, gene mutations are brought forward.

In these tautomers the mobile hydrogen atom Is changed in their position cause of which the tautorneric form show abnormal behavior. The normal pairing is A, T and T, A and G, C and C, 0

The tautomeric pairing shows A-C and G-T etc.

Unusual pairing during replication: Transition G-C to A-T pair as a result of in-corporation of rare tautomer A’ during replication of DNA, will cause point mutation. -

During replication of DNA C, C will separate, G combines with C. But C will combine with rare tautomer At This will bring a mutation.

This change is seen in the following way.

Some unnatural bases are chemically related to natural bases. Their chemical structure is related. But their behaviour is different.

Ex Thymine and 5 bromo uracil substances are chemically related. They are called analogs. Thymin combines with A but its analog 5 bromo uracil will combine with 0. In this way of the function they are differ. Hence by using base analogs we can bring mutations, A, I pair can be changed into G, C pair by using 5 bromouracil. When bacteria are grown on 5bromouracii medium mutations have come, Bromo uracil are Incorporated in place of T. Because of which in the replication A, T pair change into G, C pair.

Some chemical substances will bring mutations. They are nitrous acid hydroxyl amine.

When a bacterium is treated with nitrous acid A, Twill change into G,C pair. Nitrous acid will act on adenine and converted into xanthine. This xanthine will combine with Cytosine. This brings point mutations.

During replication of DNA molecule A,T G, C pair will produce. Now and then A may combine with G which is very rare. In the next replication this is corrected. This simple mistake in first pairing will convert A, T pair into (3, C pair.

Two bases of a pair will break their bond with sugar. When they unite with sugar they will show 180 degrees rotation. This cause C, C pair becomes C, C pair or A, T pair will become T, A. This is called transversion.

In this way point mutations will occur in organisms.

Polyploidy

Polyploidy in Organisms

In an organism all the characters are controlled by genes. All these genes are arranged in one set of chromosomes. If the nucleus contains only one set of chromosomes, it is called haploid state. The total complement of genes present in one set of chromosomes will be called Genome. If the cell contains two sets of chromosomes it is called Diploid. If an organism contains more than two sets of chromosomes it is called Polyploidy. Polyploidy plants may show.

Triploid state (3x)

Tetraploid state (4x)

Pentaploid state (5x)

Hexaploid state (6x)

Polyploidy is very common in plants.

Ex .Wheat variety

Triticum monococcum.2x (14 Chromosomes)

Emmer wheat - 4x (28 Chromosomes)

Triticum vulgare - 6x (42 Chromosomes)

Ex. Cotton plant.

Gosipium herbacium - 2x (26 Chromosomes)

Gosipium hirsutum -4x (52 Chromosomes) .

(Thus ploidy is reported in many cultivated plants).

On the basis of the source of chromosomes two types are recognised

 

a) Autopolyploidy  b) Allopolyploidy.

 

Autopolyploidy In this polyploidy condition, all the sets of chromosomes will come from same species.

The mutant verities of Oenothera are really autopolypboidy plants. In Oenothera 4x condition is seen. It contains 28 chromosomes. Because of this tetraploid condition, it becomes tall and its vegetative parts are thick and big. Tetraploidis shows aggressive characters. They can grow in different environment. They can grow in less favourable areas.

 

Allopolyploidy : It is common than autopolyploidy. In nature allopolyploidy can be seen. In this polyploidy plants, the genomes will come from different species. Such a plant is called allopolyploid.

 

Aneuploldy In polyploidy the set of chromosomes will increase.

But in some individuals one are two chromosomes will increases or decreases in genome. It is called heteroploidy or Aneuploidy. Such condition is obtained from diploid individuals.

a) Monosomic : In the diploid number of chromosomes one chromosome will be less. It is called monosomic. They are common. (2n -1)

ex .In Nicotiana tobacum now and then monosomics are obtained.

Ex. In human beings Turners syndrome is good example. He shows 45 chromosomes.

 

6) Nullisomic : In this diploid individuals one homologous pair of chromosome will be ‘missing. Such individuals are sterile (2x - 2)

c) Trisomic: Now and then one chromosome will be extra (2x -1)

ex. Klinefleter,s syndrome of man (47 chromosomes) Trisomics are unbalanced organisms.

  

c) Tetransoinice In a diploid organism if one homologous chromosomal pair is added, it is called tetrasomic. (2x + 2)

Thus if the number of chromosomes of genome will be changed it is called Aneuploidy. Because of non-disjunction of chromosomes during meiosis aneuploidy is developed.

Chromosomal aberrations -Structural change of chromosomes

Chromosomal Mutations-structural changes

Chromosomal aberrations The structure of chromosome may change. Such changes are called aberrations. They are four types.

I) Deletion, 2) Duplication, 3) Inversion, 4) Translocations.

Chromosome is an assemblage of genes arranged in a linear order. In a chromosome the number and position of genes are fixed. Hence if the position of gene is changed, in chromosome, it will change the phenotype. The structural change of a chromosome is inherited. Previously these changes are considered as chromosomal mutations. Now days they are called as chromosomal aberrations.

 

Deletion in Chromosomes

Sometimes the chromosome may lose a fragment Hence one or few genes are lost. This is called deletion. In a A, B, C, D, E, F, G,  chromosome  because of which some intrinsic, or extrinsic cause a small fragment may lost. The chromosome becomes A, B, ,E, F G. It has lost G. fragments. Hence it is called Deletion. Deletions are rare. Deletion may occur on one chromosome of a homologous pair. Then the recessive genes may be expressed. In this way phenotype characters are changed. They are hereditary. This is called Deletion. Ultraviolet rays bring deletions.

 

Duplication in Chromosomes

It is opposite of deletion. In meiosis as a result of Unequal crossing over a small fragment is added more to the chromosome.. This is called Chromosomal Duplication. In a chromosome A, B, C, D, E, F, (3, Fl.- If A, B is added then the chromosome will become A, B, A, B, C, 0, E, F, G, H this Is called duplication. Usually the additional genes will come from homologous chromosomes.

The appearance of bar eye character in Drosophila. Morgan and Strutevant described the bar locus in the salivary gland chromosome of Drosophila.

 

 

 

 

Inversion in Chromosomes: It is another type of chromosomal rearrangement. in this type the order of genes in the chromosome will change, in ABCDEFG chromosome because of inversion the gene order is changed as ABEDCFG. This is called inversion. This may be brought forward by x-ray radiation.

 

Translocations in Chromosomes

Sometimes a segment of one chromosome will break, and is transferred to -another non-homologous chromosome. Such transfer is called  translocation. It is called segmental inter change. It is resulted because of illegitimate crossing over. In general X-rays and other ionizing radiations will bring chromosomal aberrations.

also know more about Polyploidy and neo Darwinism

Neo Darwinism

SYNTHETIC THEORY OF EVOLUTION

Darwin’s theory of natural selection was accepted. The strong supporters of Darwinism are Wallace, Huxley, Haeckel, and Weismann. In the light of recent researches the theory was modified. Several experimental evidences have gone in favor of Darwinism. Basing on ‘those facts and statically data a synthetic theory of evolution was proposed. This is modified theory of Darwinism. This is called Neo-Darwinism. The ideas of Darwin were taken into consideration, But the meaning of those ideas were very much changed. Neo-Darwinism is the simple reconstructed Darwinism, like the old wine in a new bottle.
According to Neo-Darwinism the following factors operate for the formation of new species.

a) Variations

b) Mutations

c) Natural selection

d) Genetic drift

e)Isolation  of species.

Over production, struggle for existence, and universal occurrence of variation will take place as usual. But in the synthetic theory the formation of variations and mutations were discussed with experimental evidence for evolution which Darwin was unable to explain. Hence synthetic theory of organic evolution was more appropriate:

a) Variations: During Darwin’s time little was known about genetic variations. During Meosis and crossing over synapsis will take place. Because of this regrouping of genes will take place. Because of which genetic variation will appear or chromosomal aberrations will take place. The chromosomes may loose a bit or gain in a bit or order may be changed, or chromosomal bits may be exchanged between two chromosomes. These aberrations will become heritable variations.

Now and then the sets of chromosomes will increases or decrease. This is called ploidy. Because of this polyploidy heritable variations will arise they will be carried to number of generations This may result in the origin of new species.

b) Mutations: Any change in the nucleotide sequence of DNA and if one pair of nucleotides is replaced mutations will arise. These mutations are called point mutations. These are caused spontaneously in nature. They can also be brought by induction. Mustard gas, x-rays, gamma rays, electric shocks, temperature shocks etc. will bring mutations. These mutations arc rare. They are sudden and heritable. They may be harmful or beneficial. Most of the mutant genes are recessive. They can be expressed only in homozygous state.

Because of these sudden mutations new species are formed. For evolution, variations and mutations will be the raw material.

c) Natural Selection: Natural selection includes aft forces both physical and biotic factors and determine how and in what direction an organism is to change. Natural selection has no favoritism. But it is obvious that the organisms which are suited for environmental conditions will survive over power in the force of competition. Because of this better survivors are retained in the nature.

d) Genetic Drift: In small inter breeding population heterozygous gene pairs will tend to become homozygous. Because of this, disadvantage characters may be expressed and those organisms will be weeded out. Such genetic drifts are not theoretical. They operate in small populations of Islands. This genetic drift will provide a way to determine the line of evolution.

a) Isolation: In Darwin’s time nothing to known about isolation. Isolation is very important part in evolution. Usually the organisms of a population will be segregated into several populations because of physiological or geographical Isolation.

Mutations large stretches of water may separate a population in the separated groups one group may change. Because of this new species Will be developed. Thus geographical isolation will bring evolution.

The effects of natural selection in different environments will give different species.

Thus the old Darwin’s concept is re-organised with experimental proofs, New-Darwinism was proposed.

DIHYBRID CROSS

Dihybrid cross experiments of Mendel

Mendel had some Questions while doing breeding experiments. They are, do  different for different characters also segregate or seperate? Whether the genes are character would alter f the genes of another character during their inheritance together?
To answer the above Questions, Mendel did dihybrid cross. He also deduced the law of independent assortment from dihybrid cross.
1.

DIHYBRID CROSS -DEFINITION
A cross made to study the inheritance of two characters or two pairs of contrasting forms or two pairs of different alleles is known as dihybrid cross.
Example:
Crossing of pure yellow, Round seeded plant with green, Wrinkled seeded pisum plants
Character -1 - seed colour Yellow --- green
Character - 2 - seed shape Round -----wrinkled

2. PHENOTYPIC HYPOTHESIS OF DIHYBRID CROSS

A. Crossing of ‘p’ generation:
Mendel crossed pure yellow, round seeded plant with pure green wrinkled seeded plant. He got all yellow,Round seeded plants in ‘F1’ generation. These are dihybrids.
The, ‘F1’ generation showed that yellow is dominant over green and round is dominant over wrinkled forms. This dominance was allready observed in monohybrid crosses.
B. Crossing of ‘F1’ individuals:
Mendel allowed ‘F1’ di hybrid yellow, round seeded plants for self pollination. He got different types of individuals in ‘F2’.They are,
1. Yellow, Round (Double dominant)
2. Yellow,Wrinkled (Recombinant-I)
3. Green, Round (Recombinant-Il)
4. Green, Wrinkled (Double recessive)
The above ‘4’ types of inidividuals are in the ratio of 9/1 6: 3/16: 3/16: 1/16 respectively.
‘F2’ ‘dihybrid ratio led mendel to propose the law of independent assortment.

3. GENE HYPOTHESIS OF DIHYBRID CROSS

The yellow colour controlling gene is represented as ‘Y’ . The green colour controlling gene is represented as ‘y’. The round shape controlling gene is represented as ‘R’. The wrinkled shape controlling gene is represented as ‘r’.
A. Genotypes of ‘P’generation:
The genotype of pure yellow round seeded plant is ‘YY RR’. The genotype of double recessive green wrinkled seeded plant is ‘yy rr’

B. Crossing of pure ‘P’-.generation:

Pure yellow round seeded plant (YY RR) produced only one type of gametes (YR) type. All gametes carried ‘Y’ and ‘ R’ genes to the F1 generation. Pure green wrinkled seeded plant (yy rr) also produced one type of gametes (yr). When ‘YR’ type of gametes were fertilized by ‘yr’ type of gametes, all F1 inidividuals produced were dihybrid yellow round seeded plants(YyRr). The F1 generation indicates that yellow colour (Y) and round shape (R) controlling genes are dominant over green colour (y) and wrinkled shape (r) controlling genes respectively like in monohybrid crosses.

C. Crossing of’F1’dyhibrid individuals:

Mendel allowed self pollination in dihybrid yellow round seeded plants (YyRr). The ‘F1’ female produced ‘4’ types of ova. They. are YR, Yr, yR and yr. Like wise male also produced ‘4’types of pollen grains. They are YR, Yr, yR, and yr. When the above ‘4” types of ova were fertilized by the above ‘4’ types of pollen grains, Mendel got ‘4’ different phenotypes and ‘9’ different genotypes in F2 generation.
The ‘F2’generation is represented in punnet’s squares or checker board.

4. ‘F2’ – PHENOTYPIC RATIO

In ‘f2’ generation, two parent phenotypes and two recombinats are
produced. They are,
I. Double dominant (VeIIow,Round) -- 9/16.
ii. Recombinant - I (Yellow, Wrinkled ) --- 3/16.
iii. Recombinant -Il (Green, Round) --- 3/16.
iv. Double rcessive (Green, Wrinkled) --- 1/16.
In ‘F2’ generation, if we observe the phenotypic ratio of one character that is either the ratio of yellow and green or Round and Wrinkled, it is 12/16: 4/16 or 3:1 . So, in dihybrid cross, the phenotypic ratio of one character is not effected by phenotypic ratio of another.

5. ‘F2’ - GENOTYPIC RATIO

In F2 generation the ‘1 6’ individuals of 4 different phenotypes contain ‘9’ different genotypes. They are 1 :2:1 :2:4:2:1 :2:1 or (1 : 2 : j)2 . The ‘ F2’ different genotypes are
The ‘9’ double dominant yellow round seeded plants contain ‘4’ different genotypes. They are 1/9 YYRR, 2/9 YyRR, 2/9 YYRr and ‘. 4/9YyRr. The ‘3’ individuals of recombinant -I (yellow wrinkled) contain ‘2’ different genotypes. They are 1/3 YYrr and 2/3 Yyrr.
The ‘3’ individuals of recombinant-IF (green, round) also contain 2 different genotypes. They are 1/3 yyRR and 2/3 yyRr. The double recessive green wrinkled always contains one genotype only. It is yyrr. If we observe the F2 genotypic ratio of dihybrid cross, the genotypic ratio of one character is not affected by the genotypic ratio of another character.

6 LAW OF INDEPENDENDENT ASSORTMENT’

As the recombinants (yellow wrinkled and green round) were produced in ‘F2’ generation of dihybrid cross, Mendel said that every allele behaves independently. Hence all possible combinations were produced in F2 generation. It is called independent assortment. As alleles behaved independently ‘Y’ is not only inherited along with ‘R’ but also with ‘r’ . Like wise ‘y’ is not only inherited along with ‘r’ but also with ‘R’ . This independent behavour of genes lead to the formation of ‘4’ types of gametes from ‘F1’ and two recombinants in

previous topics: Monohybrid cross

Mono Hybrid Cross by Mendel

Mono Hybrid Cross

 

The principle of dominance and segregation were deduced from monohybrid cross.

MONOHYBRID CROSS - DEFINITION
A cross made to study the inheritance of one character or two contrasting forms or two different alleles is known as monohybrid cross.
Example:
Crossing of pure tall (TT) and dwarf (tt) to observe the inheritance of height character or two contrasting forms (Tall, Dwarf) or two different alleles (T t). Such crossing is called mono hybridisation and resultant ‘F1’ progeny is called monohybrids.

 

 

PHENOTYPIC EXPLANATION OF MONO HYBRID CROSS

Mendel crossed pure tall plant with pure dwarf plant. To do this, he transferred pollen grains of tall plant (male parent) to the stigma of dwarf plant (female parent or emasculated parent). It is called normal cross.
In another cross, Mendel transferred the pollen grains of dwarf plant (male parent) to the stigma of tall plant (female parent or emasculated parent). It is called reciprocal cross.
In both the above crosses, Mendel got all ‘F1’ monohybrid tall plants. Why ‘F1’ progeny is tall? This resulted him to propose the principle of dominance.

B. Law of Dominance - Definition:

Mendet proposed this principle by observing ‘F1’ progeny.
Law of dominance states that when a pair of contrasting forms (Tall, Dwarf) were crossed, the contrasting form that expressed (Tall) in ‘F1’ generation is called dominant contrasting form. The contrasting form that did not express in ‘F1’ generation is called recessive contrasting form (Dwarf).
In the above experiment tall is dominant over dwarf. What happened to the recessive allele,dwarf in ‘F1’?
To find out the answer for above Question, Mendel continued the experiment.

C. Self pollination of ‘F1’ individuals:

Mendel allowed self pollination in the ‘F1’ individuals. In ‘F2’ he got both tall and dwarf in the ratio of 3 : 1. So, the recessive contrasting form is expressed in 114 ‘F2’ generation. The appearance of recessive progeny resulted the proposal of law of segregation.
D.Observation of ‘F3’ generation:
Mendel allowed self pollination in ‘F2’ individuals to observe ‘F3’ generation. ‘F2’ dwarf plants produced all dwarf plants. It indicates that the dwarf plant is a true breeding plant. 1/3 ‘F2’ tall plant produced only tall plants. It indicates that 1/3 among ‘F2’ tall plants is a true breeding plants. 2/3 ‘F2’ tall plants behaved like ‘F1’ by producing tall and dwarf in the ratio of 3:1.

E. Law of segregation:

It was explained by Mendel by observing recessive progeny of ‘F2’ generation. 
Law of segregation states that the two alleles of heterozygous or monohybrid or ‘F1’ are seperated during gamete formation. Hence gametes are always pure. The law of segregation is also called law of purity of gametes. The gamete receives either dominant or recessive allele but never both. In ‘F1’ hybrid tall, the alleles of tall and dwarf seperated and enter into two different gametes.

Mendel,s Laws of Inheritance

The mechanism of inheritance was discovered even before the discovery of hereditdry material, D.N.A. The way of transmission of characters from one generation to another generation was first demonstrated by Gregor Johann Mendel in 1 866. He said that every cell of an organism contains two factors for each character. The factors seperate during gametogenesis. Now the Mendel factors are called genes. As the mechanism of inheritance was first discovered by Mendel, he is considered as the father of genetics. He worked on pisum plant for about ‘8’ years from 1856 to 1864. Mendel published his findings in the magazine proceedings of the natural history society of Brunn in 1866. But his findings were unnoticed until his death. But in 1900 the Mendel’s laws were rediscovered independently by Devries, Correns and Tsechermak. Hence these three scientists are considered as rediscoverers of Mendelism or principles of inheritance.

Mendel selected pisum plant for his experiments- 

i. As it is suitable for easy cross pollination 
ii. As it is a naturally self pollinated plant.
iii. As it isan annual plant.
iv. As it has many contrasting forms.

MENDELISM
The principles of inheritance or Mendel’s laws are three. They are
i. Principle of dominance
ii. Principle of segregation
iii. Principle of independent assortment.
These were deduced by monohybrid and dihybrid crosses.

Previous topic:Hybridisation

Hybridisation-Principles

Crossing of two genetically different individuals is called hybridisation. 
Example:
i. Crossing of homozygous tall (U) and dwarf (if) pisum plants.
ii. Crossing of homozygous black (BB) and white (bb) Guinea pigs.
Hibridisation experiments were conducted even before Mendel by
German scientist Kolreuter on tobacco plant. But kolreuter failed to propose principles of inheritance because he took all the characters in a plant as a single unit. Why Mendel succeeded in proposing principles of inheritance?.
1. His success mainly depends upon the selection of pea plants. . They sharply differ in few characters.
2. The plants are annuals and produce many generations within a short span.
3. The plants are self pollinated. The flowers facilitated cross pollination also. The cross pollination is carried out by removing anthers from the flowers of female parent. by emasculation.
4. He studied the inheritance of one character at a time in the begining and later two or more.
5. Mendel continued his experiments upto ‘F3’ generation (F = Filial or daughter generation)
6. He analysed the results statistically which enabled him to derive the numerical ratios.
7. “Selection of pure breeding strains as the starting material was important for his success”.
8. Mendel conducted experiments on seven characters or traits, each of which exists in two forms (dominant and reccessive)
10. ‘F’ generation
It is the filial generation produced due to hybridisation. In hybridisation experiment, first produced progeny is called ‘F1’ (1 - first ; f - filial) generation and second produced is called ‘F2’ (2 - second; F-filial) generation.
11. Hybrids
The first progeny (‘F1’ generation) produced due to hybridisation are called hybrids. These are always heterozygous.

previous topic:Genetics-basics

Genetics-Basic Terms

Genetics-introduction

It is a well known fact that like begets like. It means that the living things tend to produce offspring's that resemble them. Human beings resemble each other as they are produced by  human beings only. Why children resemble to each other? It is mainly due to inheritance . or heredity.
 

 

Inheritance or Heredity
It is defined as the transmission of characters from one generation to successive generations. Though children resemble their parents, they are not identical. No two human beings are identical except identical twins. Why individuals differ each other? It is due to variations.
 

 

Variations
The variations are defined as the differences between living organisms. These are raw materials for evolution. Variations are two types. They are acquired and hereditary variations. Acquired variations are developed in the life time due to changes in the environmental factors and food. These are never transmitted to the children. The variations which are formed by birth are called germinal or hereditary variations. The variations are formed only in sexually reproducing organisms. In asexually reproducing organisms, the parent genetic material is transmitted to the offspring’s without change. Hence variations are absent.

GENETICS
The study of heredity (similarities) and variations (dissimilarities) together called genetics.
  Gene
‘ It is a part of D.N.A that controls the protein synthesis. Mendel called character controlling unit as a factor. But the word gene was first used by Johannsen.
Phenotype
Physical appearance of an individual is called phenotype.
Examples :YeIlow seed colour, green seed colour in pisum plant.
Normally one character contains two phenotypes. The seed colour in pisum plant is character. The yellow and green seeded individuals are the phenotypes of character seed colour.

Allelomorphs - The contrasting forms of a character are called allelomorphs.
Examples:
i. The tall and dwarf plants in pisum are allelomorphs of character height.
ii. The Black and White colours of Guinea pig are the allelomorphs of character skin colour.
Alleles
The two genes that lie on same locii of two homologous chromosomes are called alleles. Normally alleles are represented as english alphabets. The dominant allele (gene) is represented as capital letter of the first letter of dominant contrasting form. The recessive allele is represented as small letter of the first letter of dominant contrasting form.
Examples:
i. The tallness controlling allele in Pisum plant is represented as ‘T’ and dwarfness controlling allele is represented as ‘t’.
ii. The black colour controlling allele in Guinea pig is represented as ‘B’ and white colour controlling gene is represented as ‘b’. Normally every individual contains two alleles for a given character. Where as the gamete always contains one allele for a given character.

Genotype .
Irrespective of the phenotype, the genetic representation of an individual for a given character is called genotype.
Genotypes are two types for a given character.
A. Homozygous:
In this the two alleles of a given character are identical. The homozygous individual receives identical genes from both the parents. The homozygous individuals are two types. They are homozygous dominant and homozygous recessive.
Examples : i. Homozygous tall (TT) pisum plant. ii. Homozygous recessive dwarf (if).
B. Heterozygous:
In this the two alleles of a given character are different. The heterozygous individual receives different genes from the parents.
Examples: i. Heterozygous tall pisum plant (Tt)
ii. Heterozygous black Guinea pig (Bb)

Fish Culture and Rearing Methods

FISH CULTURE AND REARING  TYPES

Fish culture is of different types viz.,
1. Composite fish culture 2. Cage culture 3. Integrated fish culture 4. Lacustrine fisheries 5. Sewage fisheries

1. Composite Fish Culture
Maximum exploitation of the aquatic resources through the introduction of selected varieties of compatible fish to achieve high productivity is called composite fish culture or polyculture.
Composite fish culture has been in practice in our country since ancient times. The combined efforts of Central Inland Fisheries Research Institute (CIFRI) and Indian Council of Agricultural Research (ICAR) propose the following ratios of different fishes for high yielding in polyculture.

i. Grass carp, silver carp, scale carp and mirror carp in the ratio of 5: 3 : 8 : 2.
ii. Grass carp, silver carp, rohu, scare carp and mirror carp in the ratio of 5 : 3 6 8 : 2.
iii. Grass carp, silver carp, catia, rohu, scale fish and mirror carp in the ratio of 5: 3 3 : 6 8 : 2.  

2. Cage culture
In cage culture the fishes are reared in a cage made of Nylon and wooden frame and some times in a cage made up of bamboo mesh. These cages are kept in flowing waste water canals. By this method the fishes can be protected from predators and number of fishes can be culture. Fishes feed upon the food available in waste waters. The waste waters can be re utilised for irrigation. The complications in management and investment are very little in this type of culture.

3. Integrated fish culture

Culturing of fish in association with agriculture or ducks or chicks or pigs or prawns is called Integrated fish culture.
Fish wastes fertilize the crop fields while wastes from crops and poultry chicks are used as feed by fish. individually these farming methods may yield low income. But integrated farming technique yields multiple products of nutritional value and economic importance. Various types of integrated fish culture methods are in practice today. They are

i. Fish-Prawn culture:

Prawn are cultured in ponds which are meant for carp culture in this method. The excreta of carp fishes constitute food for prawns.

ii. Fish-Poultry culture:

In this type poultry farm is constructed over a platform built of bamboo sticks above the water level of the pond. This facilitates direct fertilization of the pond by droppings of chick which are rich in nitrogen and phosphorus.

iii. Rice - Fish culture:

Fish are cultured in rice fields in this method. Fishes which can live in a depth of 15cm are selected for this culture. The rice variety selected for this purpose should also develop strong root system.

iv. Fish culture in pokkali fields:
This practice is followed in kerala. Prawn, fish and rice are cultured on rotational basis in the pokkali rice fields which are influenced by tides of vembanad backwaters. Rice cultivation
is. taken up from June to September. Fish and prawn are cultured upto May.
v. Coconut or Banana - Fish Cuture:
In banana or coconut fields, the ditches or canals in between the rows of plants can be utilised for fish culture in this method. These canals are always filled with water and rich In insect population.

4. Lacustrine fisheries
The culture of fish in takes constitute lacustrine fisheries. Natural lakes of 0.72 million hectares and manmade lakes of 65 million hectares are available for fish culture in India.

5. Sewage fisheries

In many countries fishes are introduced and cultured on commercial basis in sewage canals and ponds. The sewage is used as fertilizer in culture ponds and as feed for fish.

 

previous topic-Fresh water Fish Breeds in Aquaculture

Fresh Water Fishes in Aquaculture

TYPES OF CULTURABLE FRESH WATER FISHE BREEDS

 

India occupies second position in prawn culture and seventh position in

fish culture and production.

Fresh water fishes cultured in India are (a) Major carps (b) Minor carps (c) Murrels (d) Cat fishes (e) Exotic fishes and (f) Cold water fishes.

MAJOR CARPS

Indian major carps grow fast and can reproduce even in artificial ponds. They feed upon phytoplankton, zooplankton, decaying organic matter, aquatic plants etc. Stomach is absent in the alimentary canal of major carps. Three types Of Indian major carps are cultured in fresh water ponds.

1. Catla catla:

This is commonly called catla. It is the largest carp with grayish colour above and silvery on sides. It grows to about one meter. It has broad and stout body, broad head with upturned mouth, prominent lips and elongated fins. The dorsal side of the body is more concave than ventral side. It occurs in surface water. It matures by second year. slender fish. Body is silvery but dark gray along its back.

Pectoral, ventral and anal fins are with orange tinge. It grows to about 65 cms. The fish has a small head with a blunt snout. Mouth is sub terminal, caudal fin is sharply forked. This carp is also used in culture fishery. It normally occurs in bottom waters of rivers and tanks.

3. Labeo rohita:

This fish is commonly called rohu. It has an elongated body. Head is small but it is with a prominent terminal mouth, thick lips with short barbels. Colour is bluish or brownish gray above. Scales are gray and red or black. It grows to about 90 cms. This carp occurs in column waters of all rivers and canals. The above major carps are extensively cultured in fresh water ponds and lakes of India.

MINOR CARP FISHES

The minor carp fishes grow to a size of 30- 100cm. with an average weight of 1 to 1.5 kg. Rate of egg production is very low in these fishes.

1. Labeo calbasu:

It is commonly found in fresh water ponds and tanks of India. The body is bluish green in colour with small head and folded lips. The snout consists of four black coloured long barbs. It is cultarable in ponds. It reaches to a size of 1 m and l .5 to 2 kg. in weight.

2. Labeo bata:

It is grown in compositefish culture along with other Indian major carps. It attains sexual maturity in 9 - 10 months.

3. Labeo fimbriatus:

It has folded lips and lives in deep water zone. It grows to a maximum size of 90 cm and 450 g. in weight. Red spots are present on the scales of middle row.

The other minor carp fishes are Labeo contius (pig mouthed fish), Cirrhinus cirosa (white carp) and puntius karnaticus.

MURREL FISHES

These are air breathing fishes with long cylindrical body, flattened head and protractile mouth. These can grow in fresh water ponds, irrigation canals, wells and marshy areas. They breed even before the onset fo monsoons.

1. Channa punctatus or Ophiocephalus:(Snake head)

It is a long fish with snake like body and accessory respiratory organs. As it lives outside the water also, it is commonly called Livefish. It is coloured differently. It grows to an average length of 30 - 35 cm. It is a common food fish of high demand.

2. Channa striatus: (stiped snake head)

The body is coloured dark brown with yellow bands on either side. The fish feeds on worms and insects and grows to a length of 0.9 mt. Its flesh is good for health as it does not contain cholesterol.

3. Channa marulius: (large headed snake fish)

It is also used for culture in fresh water ponds and tanks.

CAT FISHES

The cat fishes are predatory fishes. Their skin is devoid of scales. Two pairs of barbels are present on upper and lower jaw. Most of the fish body is utilized as food due to absence of scales and spines. 

 

 

 

1. Clarias batracus:

It is commonly found in brackish and fresh water ponds of India, South and West Asian countries. Head is slightly compressed and enclosed by plates. Body is brown or dark gray in colour. It is not only used as food but also for experiments in laboratories. It is provided with Accessory respiratory organ. It grows to a size of 45 cm.

2. Heteropneustes fossilis:

The head is flat with laterally compressed body. It possesses accessory respiratory organs and lives in lake kolleru of A.P. It feeds on molluscans, algae and grows to a size of 45 cm.

3. Clarias macrocephalus:

It grows to a size of 40cm.

4. Anabas testudeneus: (Climbing perch)

It grows to a length of 15cm and feeds on aquatic insects. head is triangular with wide mouth and greenish in colour

 

 

 

 

5. Etropius suratensis:

It is commonly called pearl spot due to presence of transparent patches shining like pearls. The body is greenish, light pink and possesses eight black stripes. It is regarded as a good food fish due to delicious smell. It is not a predator, builds nests during the breeding season and hence regarded as most suitable for culture in ponds and lakes.

6. Wallago attu:

It is a cat fish found in all rivers and lakes of India. Head is larger than trunk. The body is laterally compressed. Tail is extraordinarily long and slender. It grows to a size of ito 2 mt. but captured at 90 cm. length. Two pairs of barbells are present on the head. Mouth is large with large jaws having teeth for feeding on fresh water. Hence it is called fresh water shark. As it is a predator fish, it is not suitable for artificial culture.

7. Mystus seenghala:

Four pairs of barbs, elongated upper jaw, long maxillary barbs, deeply divided caudal fin are the main features of the fish.

EXOTIC FISHES

When the indigenous fishnet are not favored for culture due to economic viability, exotic breeds are selected and cultured. These fishes yield nutritious food and earn foreign exchange.

1. Cyprinus carpio: (Common carp)

This fish was imported from China and introduced into Nilagiri lakes. The growth of the fish is higher and grows to a length of 75cm and 6.5 kg weight. It breeds thrice in a year. When cultured under extensive system, the productivity was at 1500 kg/ha.

2. Osphronemus goramy: (Gowramy)

It was imported from Jawa and Maritius and introduced into fresh waters of Madras and Calcutta. The rate of growth is very slow.

3. Ctenopharyngodon idella: (Grass carp)

It grows in fresh water, polluted water and brackish water of low salinity. It feeds on aquatic weeds and used to eliminate them. It is native of Japan and China and was introduced into Cuttack waters in 1959. It can grow to a size of 0.9m in size and 7kg in weight.

 

 

4. Hypothalamychthys molitrix: (silver fish)

It was imported from Hongkong and introduced into fresh water of Cuttack  region. The mouth is located dorsally at the tip of snout, the body is laterally compressed and enclosed by small shiny scales. It feeds on the left over food particles of carp fish and grows quickly. It reaches to a length of 60 cm and weighs about 1.5 kg.

5.Tilapia mossambicus

It was imported from East Africa in 1952. The upper jaw in males is larger. It breeds even at the age of two months.

 

 

 

 

 

Cold Water Fishes

These fishes are commonly called sport fishes. These fishes include trouts and Mahseers.

1. Salmogiardneri: (Rain bow trout)

It is a north American fish introduced into rivers and lakes of Ooty and bill ranges of kerala. The dorsal and caudal fins are pinkish with dark spots. It grows to a length of 1 .8m. and weigh upto 100kg..

2. Tortor: (Mahseer)

Head possess short rosiral and long maxillary barbs. Dorsal ride of the body is grayish green, lateral sides are gold and belly is silvery white. Even though it is adapted to grow well in canals but now it is cultured in reservoirs like Bakranangal. It grows to a size of 1 mt. and considered as good food fish.

3. Tor Khudree:

Snout is pointed. Body dark coloured on dorsal and lateral sides while yellow on ventrolateral sides.

4. Tinca tinca (Doctor fish):

It is the native of Europe and West Siberia and was introduced in Indian waters. It grows to a size of 40cm.

BRACKISH WATER FISHES

The great estuaries on river mouths and backwaters offer important potential for fish culture of particulate species. The fishes generally reared in brackish water include some iidigenous fish like Mugil cephalus, Chanoschanos, Etroplussuratensis,Latescalcarifer and some exotic species like Tilapia mossambica, Osphronemus goramy etc.

1. Mugil Cephalus: (Gray mullet)

Although these fishes are available on the coastal region, they enter into brackish waters and rivers. The fish grows to a length of 90cm. It is also reared in ponds in kerala and Tamilnadu. 70”/o of the body is useful as food.

2. Chanos Chanos:

This is commonly known as milk fish. The dorsal side of the body is greenish and shiny. It is mostly obtained in kerala state. It is highly used in brackish water culture and highly preferred food fish.

3. lates calcarifer

It is commonly known as perch. It is found in sea water, brackish waters and also in large rivers. The dorsal side of the body is dark greenish while the ventral side is shiny, It grows to a size of 60cm and may reach to a size of 150cm. It is also highly preferred as food fish.

Marine Culture (Mariculture) in India

Culture of marine fishes in coastal waters is in its infancy. There are great potentials of sea water for culture of fish, prawns, pearl oysters and mussels. Such waters suitable for maricultrure are especially abundant in kerala (Pokkali fields) Goa (Khazanlands), Karnatata (Kharlands) and West Bengal. Experimental achievements have paved way forcommercial culture of Sardinella longiceps, Sillago sihama, Anguilla bicolor, Chanos chanos and Mugil cephalus.

Salt pans at Neellaravu and Shimunipatnarn (A.P.) are especially useful for culture of prawns and fishes on commercial basis. The culture may be taken up during the period when salt manufacture is suspended that is between June and December.

Oceans cover 71% of earth’s surface providing rich source of water for fishing. It was estimated that the total marine catch was at 86 million tonnes only in 1986. Marine fisheries are largely of capture type with inshore or coastal and offshore or deep sea fishing techniques.

Principal fisheries of west coast are sciamids, polynemids, clupeids, pomfrets, sharks, rays, bombay ducks, sardines, soles, mackerels and anchovies.

Principal fisheries of east coast are clupeids, cat fishes, eels, sears, flying fishes, perches, silver bellies, sharks, pomfrets, rays and skates.

Deep sea fishes include Eleutheronema, Polydactylus, Otolithoides, Pamphus, Leiognathus etc.

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